The official said that Rossi — a two-time candidate for governor — is hiring staff and is preparing to make his widely anticipated campaign official on Wednesday. The official spoke on the condition of anonymity to discuss private campaign plans.

Republicans had been courting Rossi and national Democrats have been criticizing him in hopes of keeping him out of the Senate race or rough him up by the time he's officially in.

But Rossi has been in no rush to start campaigning ahead of the Aug. 17 primary. Complicating the GOP effort has been Sarah Palin's endorsement of another GOP candidate, former football player Clint Didier.

The late start means Rossi will have to aggressively tap his supporters to raise the money needed to mount a viable campaign against Murray, who won her last campaign by a comfortable margin.

Rossi is best known politically as a runner-up, losing governor's races to Democrat Chris Gregoire by a handful of votes in 2004 and a wider margin in 2008. But some recent polls have suggested a close race with Murray, and national Republicans have been recruiting Rossi for weeks.

Mary Lane Strow, a consultant of Rossi's who was his spokeswoman in the 2004 race, refused to confirm or deny Rossi's entrance into the race, other than saying that he would make some type of a statement midweek.

Murray was first elected to the Senate in 1992's Year of the Woman. She ran that race as an unassuming "mom in tennis shoes" but has since climbed to the No. 4 spot on the Democratic leadership team. She faced a relatively weak Republican field before Rossi entered the race and had almost $6 million in her campaign account as of March 31.

As the state's senior senator, Murray has aided powerful homegrown interests such as the Boeing Co., which is the state's largest private employer. She also has delivered federal spending to all corners of the state and is known for a special focus on veterans' issues.

"With every campaign Sen. Murray has ever run, she's not worried about who's running against her," said spokeswoman Alex Glass. "She's going to continue to do her job to get up and fight for working families of Washington state every single day."

Democrats have been anticipating a Rossi campaign and began attacking his background in real-estate investment weeks before he officially entered the race.

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